1. Field of the Invention
The present invention relates to a liquid crystal display device having switching elements such as thin film transistors (hereinafter referred to as "TFTs") which controllably switches pixels on/off for displaying images.
2. Description of the Related Art
A known liquid crystal display device 100 has an equivalent circuit such as that shown in FIG. 5. FIG. 6 is a cross-sectional view of the liquid crystal display device 100. The liquid crystal display device 100 will be described with reference to FIGS. 5 and 6. The liquid crystal display device 100 includes a pair of glass substrates 101a and 111a, i.e., an active matrix substrate 101 and a counter substrate 111, provided so as to oppose each other with a liquid crystal layer 113 interposed therebetween. The active matrix substrate 101 includes TFTs 23 as switching elements and pixel capacitors 22 arranged in a matrix. Each pixel capacitor 22 includes an liquid crystal capacitor 22a and a storage capacitor 22b. A gate electrode 103a of each TFT 23 is connected to a gate signal line 103, which carries signals for driving the TFTs 23, A source electrode 104 of each TFT 23 is connected to a source signal line 108, which carries data signals, e.g., a video signal. The gate signal lines 103 and the source signal lines 108 are provided so as to cross each other. A drain electrode of each TFT 23 is connected to a pixel electrode 106 and one of the two terminals of each storage capacitor 22b. The other terminal of the storage capacitor 22b is connected to a pixel capacitor line 25 which is connected to a counter electrode 112 formed on the counter substrate 111.
As shown in FIG. 6, the active matrix substrate 101 and the counter substrate 111 are provided so as to oppose each other with a liquid crystal layer 113 interposed therebetween. The TFT 23 includes the gate electrode 103a, a drain electrode 105 and a source electrode 104, each of the three electrodes being isolated from one another by a thin film semiconductor layer 102. An insulating film 110 is provided on the glass substrate 101 so as to cover the TFT 23. The insulating film 110 has contact holes formed in the depth direction therethrough. A conductive layer 109 and the source signal line 108 are provided on the insulating layer 110 in such a manner that a portion of the conductive layer 109 and a portion of the source signal line 108 fill the contact holes. The conductive layer 109 is electrically connected to the pixel electrode 106, which is also provided on the insulating layer 110, whereby the pixel electrode 106 is electrically connected to the drain electrode 105. An alignment film 114 is formed on each of the active matrix substrate 101 and the counter substrate 111.
In the conventional liquid crystal display device 100, a black matrix (not shown) having openings each corresponding to one pixel is provided on the counter substrate 111 for shielding regions of a display area of the active matrix substrate 101 except for the pixel electrodes 106. The display area of the active matrix substrate 101 is defined by the outer sides of the outermost pixel electrodes 106 arranged in a rectangular shape on the active matrix substrate 101. With this configuration, the black matrix needs to have an attachment margin for attachment of the active matrix substrate 101 having the pixel electrodes 106 to the counter substrate 111 having the black matrix. Consequently, the periphery of each opening of the black matrix is placed inside the periphery of each pixel, whereby the aperture ratio of the liquid crystal display device 100 can be restricted.
In order to solve such a problem, it has been proposed, in lieu of providing a black matrix, to block light incident on regions of the display area of the active matrix substrate 101 except for the pixel electrodes 106 using the signal lines 103 and 108 so arranged as to partially overlap the peripheries of the pixel electrodes 106. In this case, the aperture ratio of each pixel is defined by two adjoining gate signal lines 103 and two adjoining source signal lines 108. Thus, it is possible to realize the highest possible aperture ratio for an active matrix substrate. However, the gate signal line 103 and the source signal line 108 for one pixel each run along only one side of the pixel. Consequently, no signal lines exist on two sides of the display area. Therefore, light incident on two sides of the display area is not blocked.